Elemental Maps showing the distributions of calcium, potassium and sodium have been obtained from freeze-dried ultrathin cryosections of rapidly-frozen mouse cerebellar cortex. The molecular layer of the cerebellum wa simaged by emission of characteristic x-rays with the use of an energy dispersive spectrometer on the BEIB analytical electron microscope. Data were obtained from resting or from repetitively stimulated tissue in order to investigate changes in ion distribution that occur on excitation. There eas particular interest in determining the location of calcium concentrations that may be involved as intracellular stores required for transmission of nerve impulses. It was necessary to establish practical limits for calcium detection in an elemental map. Since the image is obtained serially, sensitivity depends on the number of pixels, i.e. on the image resolution. In practice, concentrations of about 10-20 mmol/kg wet weight could be detected.